The present invention relates to an ink-jet recording apparatus, and in particular, to an ink-jet recording apparatus capable of recording a high quality image at high speed.
In ink-jet recording, the smaller a diameter of an ink dot forming an image is, the more an image resolution is improved. Since a diameter of a dot depends on a size of a nozzle diameter, a nozzle diameter of an ink-jet head has been miniaturized recently. However, if the nozzle diameter is made to be too small, nozzle clogging tends to be caused. Therefore, the nozzle diameter is not made to be too small, but a position of an ink meniscus in the course of ink jetting and a jetting pressure are controlled finely so that an ink droplet that is smaller than a diameter of a nozzle for jetting may be jetted. Further, a large droplet, a medium droplet and a small droplet are jetted separately from the same nozzle, or small droplets are jetted continuously from a nozzle to be united, while they are flying, to form a large droplet, a medium droplet and a small droplet, so that images with gradation are formed. In addition, there has been made progress for ink, and waterfastness and light stability have been improved remarkably, compared with dyes, when pigment ink is used. Further, by adding polymer such as latex to ink, it has become possible to form an image that is free from feathering and color mixture on a medium that is unable to absorb ink such as, for example, a PET base. Further, by using polymer dispersing agents for pigment dispersion, ultrafine-frain ink can be dispersed stably, and pigment ink having bright color as in dyes has appeared. A combination of these technologies has made it possible to obtain an image exceeding a photograph from an ink-jet recording apparatus.
However, if a diameter of an ink droplet is made small or if pigment and polymer are added to ink, ink staying in the vicinity of a nozzle opening tends to be thickened, and even after an interruption of ink jetting for an extremely short period of time, ink jetting is impossible when jetting is started again, and a weight of jetted ink droplets, ink jetting speed and a direction of ink flying are changed, resulting in a phenomenon of remarkable decline of image quality. A nozzle opening is as extremely small as 20-40 μm, and ink hardly flow and diffuse, thus, moisture and a solvent each being in a small amount are evaporated from a nozzle opening, and viscosity of ink is increased locally in the vicinity of the nozzle opening. This interruption of jetting takes place when a head is located at a standby position for printing, and a speed of carriage is increased or decreased, and in the course of printing, depending on an image pattern. In the case of ink containing latex or polymer, even when jetting is interrupted for an extremely short period of time, for example, for a period in the order of a second, moisture and a solvent each being in an extremely small amount are evaporated from a nozzle opening and a film is formed, thus, viscosity is increased suddenly. In the case of ink containing pigment, if moisture and a solvent are evaporated from a nozzle opening in the course of interruption, aggregation takes place locally, causing a fear of increase of viscosity. If the jetted ink droplet is made to be small, a speed of the ink thickened locally at the nozzle opening to be carried away by jetting is lowered, and the ink is hardly replaced by bulk ink having low viscosity, thus, jetting failure is not solved simply. Though a small diameter of an ink droplet and addition of pigment and polymer to ink have made it possible to obtain images with high image quality and high durability, nozzle clogging tends to take place even in the case of interruption of jetting for an extremely short time, on the contrary, which requires to take measures.
With respect to evaporation of water from a nozzle opening of an ink jet head, detailed studies have already been made, and for example, it is known from Mehmet Z.Sengun, IS&T NIP 13, 1997, on page 681, that when pure ink containing 95% of water and 5% of ethylene glycol is left for 10 seconds under the 15% RH surroundings, concentration of water on the surface of nozzle opening is lowered from 95% to 20%. Under the 60% RH surroundings, concentration is lowered to 40%, and viscosity is increased suddenly in both cases. When pigment or polymer is contained, deposit of solid or formation of a film takes place in a very short time, and viscosity is capable of rising suddenly. Since ink viscosity at the nozzle opening is suddenly increased locally as stated above even in the case of interruption of jetting for an extremely short time, inability of jetting or image deterioration is caused unless jetting is conducted after viscosity is lowered.
In the case of suspension of printing for a long time, it is possible to prevent evaporation of ink component from a nozzle opening by covering the entire nozzle surface with a cap. However, capping is impossible in the period of standby for printing or in the period of interruption of jetting in printing. As one of measures for the foregoing, there is a method to stir thickened ink located in the vicinity of the nozzle opening together with bulk ink in a channel and thereby to lower ink viscosity on the nozzle opening surface, by making an ink meniscus in the nozzle to vibrate finely by driving piezoelectric material slightly to the extent not to jet ink. For example, when ink in all nozzles is made to vibrate slightly so that ink viscosity on each surface of nozzle opening may be lowered, when a head is located at the standby position outside the printing area, ink jetting is possible for the first droplet and thereafter for all nozzles. Further, it is possible to prevent thickening of ink located at the surface of the opening of a nozzle which is not jetting, by applying fine vibration drive signals in place of signals for jetting on piezoelectric material of a channel that does not jet in the course of printing in the printing area. As stated above, fine vibration includes fine vibration outside a printing area and fine vibration inside a printing area. Incidentally, when printing only a rear end portion in scanning in the case of a large-sized recording medium, even when ink viscosity is lowered at the standby position by making the ink meniscus to vibrate finely, ink viscosity on the nozzle opening is increased again before the start of jetting, because there is an interval of time before the succeeding jetting. Therefore, it is necessary to apply fine vibration again immediately before jetting. However, the timing for applying the fine vibration is difficult because stable jetting is impossible without waiting until residual vibration is brought to an end after the fine vibration is applied to the ink meniscus.
Thickening behavior varies depending on a difference between pigment ink and dye ink and on ambient temperature and humidity. It further varies depending on the state of heat generation of a head, and therefore, the optimum fine vibration needs to be applied by changing an amplitude of vibration, a frequency and the number of times of repetition for various circumstances. For example, in the case of ink that is easily thickened or of low temperature and low humidity environment, fine vibration is not applied sufficiently and ink is not stirred properly. In the case of ink that is hardly thickened or of high temperature and high humidity environment, too much fine vibration is applied, and a nozzle surface is contaminated by overflowing ink, and the direction of an ink droplet to be jetted next is deflected. Therefore, it is necessary to monitor the ambient temperature and humidity, the types of ink and temperature of a head by using various sensors, to analyze printing data sent from the head to obtain frequency of jetting ink droplets, and to control fine vibration conditions in detail and apply the optimum fine vibration, which, however, is extremely troublesome.
On the other hand, there is a method wherein fine vibration is applied outside a printing area, and then, thickened ink is spewed. In this method, viscosity on the opening section is hardly increased, and fine vibration does not need to be applied in the course of scanning, because a grain of thickened ink is discarded.
The technology to lower thickened ink viscosity by applying fine vibration on a meniscus is widely known in the field of an ink-jet recording apparatus. For example, Japanese TOKKAISHO 55-139271 discloses a technology to apply signal voltage of an amplitude or a pulse width which makes an ink droplet not to be jetted from a surface of a nozzle in the course of non-recording. Japanese TOKKAIHEI Nos. 9-29996, 10-81012 and 11-300966 and TOKKAI No. 2000-94669 disclose technologies wherein an ink meniscus of an ink-jet head employing a layer-built piezoelectric material is vibrated finely. In these technologies, a one-sided rectangular wave form or a trapezoidal waveform is used (an inclination of waveform in each of rising and falling is made to be ½ of Helmholtz resonance frequency of an ink chamber or less) to vibrate a meniscus finely. In these conventional technologies, however, it is impossible to stir thickened ink effectively. Therefore, it is necessary to set the number of times for application of fine vibrations to be great. Further, a level of each voltage for each of fine vibration, jetting and ink spewing before printing needs to be adjusted, which makes the structure of drive circuits to be complicated. Further, when an inclined form is used for a drive waveform, sensitivity for voltage is lowered more compared with an occasion of a rectangular wave, and necessary drive voltage rises and power consumption is increased, which is a problem. In addition, if the number of times of application of fine vibration drive signals is not increased, sufficient effects are not obtained, resulting in a decline of the printing speed. Therefore, effective application methods by which great effects are obtained in a short period of time are sought, by utilizing a returning travel of carriage scanning.
Japanese TOKKAI No. 2000-168103 discloses a technology wherein 80-100 pulses of drive signals are applied after discontinuance of jetting for a long period of time to push a meniscus out of a nozzle, and then, application of pulses is stopped to pull the meniscus in the nozzle for jetting. In this technology, however, since the meniscus is pushed out of the nozzle gradually when 80-100 pulses of drive signals are applied, the meniscus spreads sideways up to the surface of a nozzle plate, and it takes a considerable period of time for the meniscus to be pulled in the nozzle plate. After the meniscus is pushed out, application of pulses is stopped only to restore to the state wherein the first meniscus is formed, and sufficient effects of stirring are not obtained accordingly. Moreover, nothing is disclosed about a concrete shape of a pulse.